Rob McCartney added: “The mother came running up the street, shouting for help and saying ‘Get an ambulance’.”

Paramedics who attended found another wound to Katie’s torso – which ran for around 20cm from her sternum to her stomach and had cut through skin, fat and muscle, with shorter cuts to each side.

Her clothing had been pulled up to inflict the wound, which was made with a red Stanley knife that the teenager had taken from her grandmother’s kitchen and handed to police when she was detained near the scene.

Blood staining on Katie’s green coat with a fur-trimmed hood, school trousers and school shirt all suggested she was lying on her back when she was cut with the knife.

Tests later found that Katie had been smothered and the knife wounds inflicted after her death. Katie was pronounced dead at hospital that evening.

Mrs Rough told the BBC: “We went in to see her and the rest is a blur. The next few days were an absolute blur.

“It was just the darkest. I never knew that I could feel something so terrible.

“There’s no words for how we felt. It was just absolute devastation.”

The court was told yesterday that the teenager was standing in a nearby cul-de-sac, covered in blood and carrying the blood-stained Stanley knife as she rang 999 to tell police what she had done.

The defendant appeared by video link where she denied murder but pleaded guilty to manslaughter by diminished responsibility.

This plea was accepted by the prosecution.

The judge, Mr Justice Soole, was told how the girl began suffering from mental health problems more than a year before the killing.

Mr Reeds said she had reported delusional thoughts as well as depression, self-harm and suicidal thoughts.

Girl writes plea down on paper

During the hearing the dark-haired girl appeared on the video link sitting next to a solicitor and wearing a black hoodie.

The teenager’s solicitor confirmed her name when asked by the judge.

Nicholas Johnson QC asked the court if the charge of murder could be put to the girl again and she wrote her plea on a piece of paper.

Her solicitor told the court: “I can confirm she has indicated not guilty to murder but guilty to manslaughter.”

The judge said he wanted more questions answering by the medical experts before he could pass sentence.